1. Field of the Invention
The present invention relates to a device for detecting state of health (SOH) of batteries and more particularly to a device capable of quickly mapping to a detected voltage of a battery to an instant SOH of the battery from a known curve for the SOH of the battery.
2. Description of the Related Art
Progress in battery technology and demands for energy conservation, carbon reduction and environmental protection render more and more opportunities to the use of electric vehicles. To meet the requirement of higher driving range and performance, the battery pack used in the electric vehicles is formed by multiple batteries connected in series or series-parallel to achieve desired voltage and capacity, and increasing efficiency and performing degradation analysis are the critical measures to meet the foregoing requirement.
The major energy source of electric vehicles comes from the battery packs mounted inside the electric vehicles. The capacity of energy stored inside the battery pack is an important criterion for users to consider charging the battery pack. Despite the currently growing market demand for electric vehicles, most of the existing electric vehicles are dedicated to short to medium range commuting. Users of most electric vehicles live in residential buildings or apartments. Unless any charging outlet or charging station is available in the parking area, charging an electric vehicle could become an issue. Hence, a preferable solution is to widely build battery exchange stations. When electric vehicles are running out of battery, drivers of the electric vehicles can pay for a fully charged battery pack from a neighboring battery exchange station to replace the original battery pack and the amount of the payment for the exchange depends on the remaining capacity of the replaced battery pack. The battery exchange station then needs to test the battery capacity and perform a battery degradation test of the replaced battery pack.
When a effective capacity of a battery applied in an electric vehicle is lower than 80% from the original capacity, the battery is deemed as one with serious degradation and no longer complies with the requirement of the electric vehicle. Hence, performing a battery degradation test can avoid miscalculation of an existing effective capacity of the replaced battery.
Devices in the market for capacity estimation and degradation test of batteries focus on the lead-acid batteries for vehicles, and the most common devices are battery life testers or battery testers. The battery life tester has a high-power load circuit mounted therein for a large discharging current so that the battery life tester can measure a voltage drop and thus a discharging capability of batteries. A state of degradation of the battery can be determined from the discharging capability. Because of the high-power load circuit, the battery life tester is bulky and inconvenient to be carried around. The discharge current during the test period is rather high so that the testing time of the battery should be short to avoid the effect of degradation or damage on the battery. The battery testers serve to analyze four items (voltage, cold start current, resistance and battery energy) to determine the degradation degree of a battery. Practically, the resistance (internal resistance) of a battery is measured through an AC measurement method for measuring the internal resistance of a battery to obtain a corresponding battery capacity. However, when a battery tester is measuring the internal resistance of a battery, the engine of the vehicle must be turned off for at least one hour, and only one battery is allowed for testing at a time. When multiple batteries are connected in series, it is impossible to measure the battery capacity of each battery.
Disclosed as a currently available technique for detecting state of health (SOH) of batteries, the U.S. patent publication number 20070252600 entitled “Diagnosis method for state-of-health of batteries”, discharges a constant current from a battery for a long period of time, then measures a voltage value of the battery, observes a voltage variation rate, and further determines if the SOH of the battery is abnormal. However, such technique takes a long time for the battery to discharge, the degradation of the battery is the only state that can be determined, and an instant SOH of the battery cannot be rapidly diagnosed.
Furthermore, as disclosed in the U.S. patent publication number 20090128097 entitled “Method and system for tracking battery state of health”, the system for tracking battery SOH is composed of a charger, a charging counter and a processor. When the system charges a battery to be tested and starts counting time, the capacity of the battery during the charging course is calculated and is compared with a original capacity of the battery to determine the SOH of the battery. As the calculation of the capacity involves an accumulative method, long estimation time fails to achieve fast determination and a specially designed filter is required, an instant SOH of the battery is unable to be diagnosed.
From the foregoing, most conventional devices measure battery capacity using a current method and an internal resistance method. The current method employs a specific current-discharging pattern and measures a voltage variation rate serving as a basis for determining the SOH of the battery. The drawback of the current method resides in that a long discharge time arises from complete life cycle test of the battery. The determination of SOH is limited to merely determining if the battery is healthy without knowing the health degree of the battery. The internal resistance method needs to provide an external input voltage of a battery to be tested for measuring an internal resistance value of the battery, and determines a degradation status of the battery after a calculation. The drawback of the internal resistance method resides in that the measuring device demands for higher specification, is thus more costly and is composed of many subsystems. The accumulative method continuously measures and records an electric charge of a battery during the course of discharge and compares the measured electric charge with an originally recorded electric charge of the battery to determine the SOH of the battery. The drawback of the accumulative method resides in that fast determination of SOH is impossible because a complete charge or discharge procedure is required and a specially designed filter must be provided.